1. Field of the Invention
The present invention relates to a magnetoresistive effect element (MR element) in a CPP-type structure that detects magnetic field intensity as a signal from a magnetic recording medium, and so on, a thin film magnetic head with the MR element, and a head gimbal assembly and a magnetic disk device that have the thin film magnetic head.
2. Description of Related Art
In recent years, with an increase in the high recording density of a magnetic disk drive (HDD), there have been growing demands for improvements in the performance of a thin film magnetic head. For a thin film magnetic head, a composite type thin film magnetic head has been widely used; it has a structure where a reproducing head having a read-only magnetoresistive effect element (hereinafter, magneto-resistive (MR) element), and a recording head having a write-only induction type magnetic conversion element are laminated together.
Generally, a shield layer is formed in a reproducing head to restrict an area of a medium that interferes with a reproducing element. Currently, in a conventional head structure, a first shield film, a second shield film and an MR element are connected in series without an intershield insulating layer. This structure is referred to as an MR element in a current perpendicular to plane type (CPP) structure. In consideration of the efficiency of heat dissipation and maintenance of an output, and so on, a CPP-type structure is essential to realizing a high recording density beyond 500 Gbits/in2.
A general CPP-type element with a spin valve is briefly explained below. A typical spin valve CPP-type element is formed by a lamination structure for its main layers as follows: a lower electrode layer/an under layer/an antiferromagnetic layer/a ferromagnetic layer (1)/a spacer layer/a ferromagnetic layer (2)/a cap layer/an upper electrode layer. The top most layer is the upper electrode layer, and the bottom most layer is the lower electrode layer. In the specification, a lamination layer may be described as having the above format.
A magnetization direction of the ferromagnetic layer (1), which is one of the ferromagnetic layers, is pinned in the perpendicular direction to a magnetization direction of the ferromagnetic layer (2) when the externally applied magnetic field is zero. The ferromagnetic layer (2) is generally referred to as a magnetic free layer. The magnetization direction of the ferromagnetic layer (1) can be pinned by making an antiferromagnetic layer adjacent thereto and providing unidirectional anisotropic energy (also referred to as “exchange bias” or “coupling magnetic field”) to the ferromagnetic layer (1) by means of exchange-coupling between the antiferromagnetic layer and the ferromagnetic layer (1). For this reason, the ferromagnetic layer (1) is also referred to as a magnetic pinned layer.
As mentioned above, a CPP-type element that is configured with a connection between a shield layer and an MR element through a metal is advantageous because it increases heat dissipation efficiency and operating current. In this CPP element, a smaller cross sectional area of an element has a larger resistance value and a larger resistance variation. Namely, it is an appropriate structure for a so called narrower track width. A narrower track width increases the number of tracks per inch (TPI), and it is an essential technology for increasing the recording density of a hard disk drive (HDD).
However, in view of the high frequency characteristic of the above CPP element, an increase in the resistance of such an element is undesirable. Therefore, research and development has been performed to develop a CPP-GMR element that has a spacer layer made of a low resistance material instead of having a tunneling magnetoresistance (TMR) element with a barrier that has a high resistance value.
As an example of a practical CPP-GMR element, there is a spin-valve type CPP-GMR element that is configured with the following layers in a bottom up direction: a lower electrode, a PdPtMn antiferromagnetic film, a magnetic pinned layer (pinned layer) of which a magnetization direction is pinned with respect to the antiferromagnetic film and which is configured with a CoFeB film, a spacer layer configured with a Cu film, a magnetic free layer (free layer) of which a magnetization direction varies depending on an externally applied magnetic field and which is configured with a CoFeB layer, and an upper electrode. A magnetoresistive ratio (hereinafter “MR ratio”) of this element is about 1.16% in a single spin-valve structure. This MR ratio is not large enough to generate appropriate output power in view of practical applications. This is because when a practical application of a head with an areal density beyond 600 Gbpsi is considered, the S/N (signal to noise ratio) ratio improves as the MR ratio increases. Meanwhile, with respect to a dual spin-valve structure that has a relatively large MR ratio, such a structure does not satisfy the demand for a narrower read gap because a total layer thickness of a spin-valve is large.
As one instrument for improving an MR ratio of a CPP-GMR element, a so-called current confined path (CCP) CPP-GMR element has been proposed. This structure controls the flow of a sense current so that an effect of the spin-dependent scattering associated with a material is utilized to a maximum extent. When this structure is used, the MR ratio is improved. In Japanese patent number 3293437, a technology for enhancing a GMR effect by using a CCP structure is described as a spacer layer.
In Japanese laid-open patent publication number 2003-204094, a technology providing an insulating material distributed along the interface in a spacer layer for realizing a CCP-CPP-GMR element is disclosed. For example, the spacer layer is configured with a Cu/nano oxide layer (NOL)/Cu structure.
In Japanese laid-open patent publication no. 2002-208744, a technology providing a “resistance adjustment layer” along an interface of a magnetic pinned layer magnetic free layer or a spacer layer is disclosed. The resistance adjustment layer is defined as a layer configured by mixing a conductive layer and an insulating layer that has an aperture ratio of 50% or lower by pinholes. A specific structure of the resistance adjustment layer is configured with two or more kinds of metals (for example, AlCu) in which one metal is preferably oxidized through an oxidation treatment.
It is possible to increase an MR ratio because the resistivity of a CPP-type element can be increased to a high level through the technologies discussed above. However, these technologies can be unreliable. Since a current confined path is an essential part of these technologies, and a current is concentrated at a current confined path of an electrically conductive part so that the current density of the electrically conductive part is increased, local migration occurs.
In consideration of the situation mentioned above, a group of members including inventors of the present invention has focused on use of a semiconductor material, such as ZnO, In2O3, and SnO2, as a nonmagnetic intermediate layer of a CPP-GMR element. The group determined that a CPP-GMR element made of such semiconductor materials has a high MR ratio (see Japanese laid-open patent publication numbers 2008-91842 and 2008-205438).
However, in consideration of future applications with high density recording of 2 T bpsi or more, it is predicted that an MR element, which has an MR ratio of 50% or greater when an area resistivity (AR) of the MR element is equal to 0.2 Ωcm, will be required based on a calculation of a S/N ratio. However, an MR ratio of a currently reported CPP-GMR element is still relatively low. Therefore, there is a need for development of an element structure that further improves an MR ratio.
In consideration of the situation described above, the present invention is provided. An object of the present invention is to provide a novel MR element enabling further improvements of an MR ratio and inhibiting an increase of an area resistivity (AR) to enable the MR element to be used for future high density recording applications requiring 2 T bpsi or more. It is also an object to provide a thin film magnetic head that has the MR element mentioned above, and a head gimbal assembly and a magnetic disk device that have the thin film magnetic head mentioned above.